We propose a molecular epidemiologic cohort study of all newly diagnosed Stage IIA and IIB breast cancer patients who registered at The University of Texas M.D. Anderson Cancer Center (MDACC) between January 1, 1985 and December 31, 1999, and were residents of the state of Texas (N=2,875). This cohort is characterized by extensive long-term follow-up, and equal access to "best of care" practices. It offers us the opportunity to test hypotheses regarding the role of host genetic (i.e., such as gene polymorphisms in drug and radiation sensitivity, family history and ethnicity) and exposure factors (i.e., smoking, reproductive events, BMI, HRT, co-morbid conditions) in relation to tumor phenotype, as independent and interactive determinants of risk of recurrence, distant metastasis, contralateral breast cancer, second malignancies, and/or disease-free survival. In this population, we will examine a series of promising candidate host genetic markers and suspected environmental and lifestyle factors for their added value in clinical decision making when compared or combined with standard histopathological prognostic markers (e.g., lymph node status, tumor size, histological grade). In order to address our study hypotheses, we propose the following 4 specific aims: 1) To compile a comprehensive database of epidemiologic risk factors and clinical data (including treatment, pathology, and long-term follow-up) on all breast cancer patients seen exclusively at MDACC for Stage II disease. 2) To evaluate baseline epidemiologic profiles as independent and inter-related determinants of risk of recurrence, distant metastasis, contralateral breast cancer, new primary malignancies, and disease-free survival. These profiles will include comprehensive information on the use of tobacco, reproductive factors including ovarectomy and the use of HRT, body mass index (BMI), family history of cancer, age and ethnicity in relation to standard histopathology markers. 3) We will evaluate the interaction between these epidemiological factors and a) host and b) tumor molecular markers that show a relationship with disease prognosis and patient events. Specifically, in our sample set we will evaluate: the tumor specific expression of HER2-neu, Ki-67, ER, PR and p53, using standardized immunohistochemical analysis, as informative markers of tumor phenotype and patient performance. Inherited variability in genes that modulate host sensitivity to therapeutic agents, including the glutathione sulfotransferase (GSTpi, GSTM1, GSTT1); enzymes important in defending against chemical injury by catalyzing conjugation of reactive electrophilic molecules with glutathione, and an allele variant in the multi-drug resistance gene MDR-1 whose products, P- glycoprotein, is strongly linked with chemotherapeutic resistance. Our fourth aim is to construct statistical models to determine which marker or combination of markers and host factors (genetic and lifestyle) are better predictors of patient performance than the commonly used histopathologic methods.